The present invention relates to a control technology of optimizing the quality of a signal output from an analog circuit.
Analog circuits find difficulty in keeping its quality constant. The reason is that analog circuits are affected by not only an absolute variation in the characteristics of transistors constituting the circuits but also a relative variation in the characteristics between different transistors. In other words, the characteristics of an analog circuit are directly affected by the characteristic distributions of transistors constituting the analog circuit. Therefore, in system LSI including both analog circuits and digital circuits, if the characteristic distributions of transistors greatly fluctuate due to an unexpected variation in fabrication process, the yield of the system LSI will be greatly reduced. Analog circuits are also affected largely by the use environment. That is, the circuit characteristics greatly change with the influences of the operating voltage, the ambient temperature and the like, and this may adversely affect the entire system using the analog circuits.
To solve the above problem, systems permitting automatic calibration of analog circuits are known. As such systems, generally known are a system in which optimization is made at the startup of an analog circuit and no more made thereafter and a system in which optimization is made at fixed intervals.
A related technology is disclosed in “An Accurate Center Frequency Tuning Scheme for 450-kHz CMOS Gm-C Bandpass Filters”, IEEE Journal of Solid-State Circuits, Vol. 34, No. 12, December 1999, pp. 1691-1697, for example.
However, the systems described above have a problem that if the characteristics of an analog circuit to be calibrated change with a fluctuation in temperature and a fluctuation in power supply voltage, the subsequent circuit characteristics will no more be optimal.